Sustainable Farming

Managing liquid manure through micro-aeration

Adding oxygen & carbon to liquid manure enables microbes
to feed on the ammonia in the manure. The result is a product
which not only does not stink, but enhances rather than destroys
soil life.

During the past 18 years I have been actively involved in the
development of liquid manure composting. Back in Switzerland, we
had conducted several farm-scale experiments in a number of
different situations and with a variety of types of aeration
equipment. It was soon apparent that we were dealing with a very
demanding task and we experienced a lot of confusion and
frustration!

One has to be aware of the complexity of such a process. The
wide spread in results is caused by a huge number of variables
affecting the final outcome of the process. The quality of the
finished product also has to be evaluated from many different
points of view.

Liquid manure can be one of your most valuable resources to
enhance soil fertility or it can be one of your biggest problems.
When stored under anaerobic conditions it gives off harmful gases
such as ammonia, methane, and hydrogen sulphide. Sludge build up
makes it difficult to clean out.

It can also enrage neighbours to the point where the farmer
may be sued. If it's spread on the land in this state, it's doing
more harm than good.

The poisonous gases and toxic substances endanger the life in
the soil, resulting in damage to soil structure and plant growth.
Due to a very unbalanced nutrient content, massive losses -
especially of nitrogen - will occur. Some of this is by
leaching (nitrates and nitrites) and some of it is in gaseous form
(ammonia).

The alert observer will notice after spreading such manure,
that depending on the potency/concentration of such a slurry,
considerable numbers of dead earthworms will appear. This should
be an alarming sign that something has to be done! This
biological disaster is caused by a combination of drowning the
worms and gassing them out with lethal doses of ammonia (which,
incidentally, also kills people). Of course, the earthworms
belong to the more visible members of the soil life; just imagine
what it will do to the rest of the countless different species
contained in the soil.

What can be done with liquid manure?

Just like in a solid manure composting process, we want to
promote an aerobic fermentation. To achieve such a decomposition
in a liquid mass, we employ a micro-aeration technique to
incorporate minute bubbles of air throughout the entire storage
facility of liquid manure during calculated time periods.

The bubbles are so small that few are seen to reach the
surface. A cover (natural or artificial of straw, compost, etc.)
should be maintained at all times to prevent the escape of
ammonia during the process. A light milky whine foam may be
noticed at exposed areas. This is because the air is either
absorbed into the liquid, being used immediately by bacteria, or
clings to particles in the liquid.

The micro-aeration unit itself consists of a submersible
diffusion pump which injects the air by creating a vacuum at the
very bottom of the tank. The unit is operated on a timer, running
a few minutes every half an hour, depending on various factors.
The objective is to supply an adequate amount of oxygen to
enhance the right level of microbial activity in relationship to
available carbon and nitrogen in the slurry.

A careful balance

The aerobic bacteria which break down carbon material, feed on
the ammonia, thereby tying it up biologically in their bodies
(protein compounds) and stabilizing H. So the balance between
carbon, nitrogen and air is essential and depends a lot on the
type and consistency of the slurry.

Manure from cattle on high grain rations, as well as pig
manure, often lacks carbon and also has a higher ammonia content
than slurry from animals on a high roughage diet. This influences
the C/N ratio dramatically. Ideally we would like to see this
ratio between 12 and 14:1 to start out with. It will narrow down
during the process to 3-4:1 in the finished product.

Carbon-source materials, such as finely ground-up straw or
partially decomposed manure, are best suited to solve the ratio
imbalance. It is a good practice to provide a source of readily
available carbon such as feed grade molasses at a low rate each
day of the processing period. This measure has an explosive-like
impact on the growth of the microbial activity in the slurry.

If the bacteria are growing with the right proportions of the
three key ingredients, they stabilize the nitrogen and other
nutrients so that the liquid may contain as much nitrogen and
other nutrients as it did when it was fresh from the animal.

Nutrient preservation is the main objective and as a result
the liquid manure will not have the unpopular offensive smell of
conventionally harmful slurry. Because up to 85% of the nitrogen
(compared to 3560% in untreated slurry) is in its organic form,
the liquid can be top-dressed on plants without causing burning of
tissue. K is more stable in the soil and enhances rather than
destroys soil life. This manure can effectively contribute to
humus production, a key ingredient in a healthy soil.

Reprinted from the Ecological Farmers Association of Ontario
Newsletter.